1. Field of the Invention
The present invention relates generally to improvements in fiber optic acoustic sensor array systems and more particularly pertains to a new and improved method and apparatus for detecting the signals from an array of fiber optic interferometric sensors for determining changes in a physical parameter measured by the individual sensors.
2. Description of Related Art
Mismatched fiber optic interferometers are commonly used as sensing elements in fiber optic sensor arrays for measuring changes in parameters such as fluid pressure, acceleration or magnetic field intensity, for example. Such sensing elements measure the phase delay between two optical paths having unequal path lengths. Typically, in time division multiplex (TDM) systems, a modulated optical signal is input to the sensor array and various demodulation techniques have been proposed and are used for correlating the signals from the array of sensors that produce the signals.
Common to all demodulation methods for fiber optic interferometric arrays, is the acquisition of an in-phase term proportional to the cosine of the interferometer phase shift and a quadrature term proportional to the sine of the interferometer phase shift. The sine of the sensor phase shift, is known as the quadrature term Q; and the cosine of the sensor phase shift is referred to as the in-phase term I. The angle of the phase shift is determined by calculating the ratio of Q/I, which is the tangent of the sensor phase shift. The amplitudes of the sine and cosine terms must be set equal by a normalization procedure to ensure the successful implementation of an arc tangent routine to find the sensor phase shift.
An interrogation method called differential delay heterodyne is one of a variety of methods used for fiber optic acoustic sensor array systems. It uses an in-line Michelson sensor time division multiplexed (TDM) array structure and a compensating interferometer section.
An example of such a system is shown in FIG. 1 as having a continuous wave laser 11 supplying signal energy to a pair of acousto-optic modulators 13, 15 that act as optical gates to produce pulses like the two optical pulses 1 and 2. Each acousto-optic modulator puts a unique frequency shift on the light energy from laser 11. This provides, for example, pulse 1 at a frequency shift of 105 MHz and pulse 2 at a frequency shift of 95 MHz for a difference frequency of 10 MHz between them. A delay coil 17 in series with the acousto-optic modulator 13 creates an optical path length difference between the pulse signal paths for the signal's output from the two acousto-optic modulators 13 and 15.
These output pulses travel down optical transmission line 16 in direction 43 to an in-line Michelson array with two hydrophone sections 19 and 21 located between mirrors 23(A), 24(B) and 25(C). Mirrors 23(A) and 24(B) are coupled to transmission line 16 by tap couplers having appropriate coupling ratios. The spacing between the three mirrors 23(A), 24(B) and 25(C) is selected to produce a reflected sequence of four pulses 35, 39, 41 and 37 traveling out of the Michelson array, in direction 45.
Return pulse 35 is effectively pulse 1 reflected from mirror 23(A). Pulse 35 can be labeled 1(A). Pulse 39 is an interference pulse signal that is a combination of pulse 1 reflected from mirror 24(B) and pulse 2 reflected from mirror 23(A). Return pulse 39 can be labeled (2A/1B). This pulse contains acoustic phase information from the hydrophone section. Pulse 41 is another interference pulse containing information from the combination of pulse 1 being reflected from mirror 25(C) and pulse 2 being reflected from mirror 24(B). Pulse 41, therefore, could be labeled (2B/1C). Pulse 37 is simply pulse 2 reflected from mirror 25(C). (Pulse 37 could be labeled pulse 2C.) The middle two pulses, 39 and 41, are the interference pulses that contain acoustic phase information 40 and 42, respectively from the two hydrophone sections. The system could be expanded to any number of hydrophones, so that for N hydrophones there are N+2 return pulse signals.
Pulses 35 and 37 contain no useful information.
These return pulse signals are demodulated in a receiver that contains a local oscillator 31 set at the difference frequency between the modulator frequencies of the two acousto-optic modulators 13 and 15. The local oscillator signal is mixed with the output signal from the photodiode detector 27 in mixer 29 to produce the cosine I and sine Q components of the optical signal. These cosine and sine components of the optical signal are then processed in the demodulator 33 to produce the signals representative of the change in parameter measured by the Michelson array.
The above example is only one of many available methods of demodulation known in the prior art. Other methods are shown and described in U.S. Pat. No. 6,154,308, U.S. Pat. No. 6,122,057, and U.S. Pat. No. 5,917,597. These patents show other examples of demodulation methods for fiber optic sensor arrays.
The present invention provides an improved method and apparatus for demodulating signal's from a fiber optic sensor array, by eliminating the need for a mixer and local oscillator, as shown in FIG. 1.